High molecular weight linear polyester compositions based on bisphenols have been shown to be useful in the preparation of films and fibers. These compounds, when molded into useful articles using conventional techniques, offer properties superior to those articles molded from other linear polyester compositions.
Bisphenol polyesters can be prepared by three condensation techniques, i.e., melt homogeneous and interfacial condensation techniques. Melt or bulk polymerization is the simplest method and in this technique the reactants are charged into a vessel and heated. Homogeneous or solution polymerization generally offers a better rate of reaction and temperature control than the melt process since solubility of all reactants in a common solvent permits the reactants to be more thoroughly dispersed and the resulting product is more conveniently handled. In the interfacial method, the reactants are dissolved in solvents which are immiscible with each other and their reaction takes place at the interface of the solvents.
Of the three condensation techniques, homogeneous or solution polymerization is the least often used for two reasons. The first is that the interfacial technique provides all of the advantages of the homogeneous technique and additionally provides means for maintaining the concentration of the reacants in the reaction zone at a constant level. The second reason is that it is difficult to produce high molecular weight polymers by solution polymerization while such high molecular weight products are easily obtained using either the melt or interfacial techniques. Thus, in solution polymerization, the product rarely has an intrinsic viscosity in excess of 0.6 deciliter per gram of polymer when measured in a solution of symmetrical tetrachloroethane at 30.degree. C., and indeed, most polymers produced by this method have intrinsic viscosities of less than about 0.4 dl/g. The intrinsic viscosity is, of course, a measure of the molecular weight of the product and as the molecular weight decreases, the polyesters become more brittle and lose strength. Thus, the molecular weight of the polyesters produced by the solution process are usually not high enough to produce a polyester having good impact strength.
The interfacial technique has several disadvantages. One is the possibility of hydrolysis of some of the diacid chloride and formation of carboxylic acid groups which can then react with more diacid chloride forming anhydride linkages in the growing polymer. Exposure to moisture during processing and/or use can result in hydrolysis of the anhydride linkage which could seriously degrade polymer properties. In a solution method, moisture can be excluded by careful drying of all ingredients and solvents by known methods such as distillation, azeotropic distillation of drying of solids in suitable equipment such as vacuum ovens. Also in a solution polymerization method, if the hydroxyl-containing compound reacts very rapidly with the diacid chloride it may not be necessary to eliminate the very small amount of water that may be present as an impurity. In an interfacial process, large amounts of water are always present and thus the probability of reaction with the dihalide is increased. Another disadvantage of the interfacial process is the low yield per batch of 3%-5% polymer solids while the solution method affords up to 11% polymer solids per batch.
In the solution condensation of a bisphenol and an aromatic dicarboxylic acid halide, it is known to add the dihalide to the bisphenol or glycol or to mix the two reactants followed by addition of a suitable catalyst, and in some instances to initially employ one diol so as to form a prepolymer and thereafter add the second diol. See, e.g., Korshak et al, J. Poly. Sci., A-1, 11, 2209 (1973). The addition of the aromatic dicarboxylic acid halide to the hydroxyl-containing component is standard procedure.
We have now discovered that despite the reasons for the standard practice in the art of adding the diacid halide to the hydroxyl-containing component, if the hydroxyl-containing component is added to the diacid halide, solution polymerization will yield high molecular weight polyesters which have a low melt viscosity and, additionally, the yield per volume of reactants and sovlents can be improved. We have also found that the high temperature prepolymerization step generally employed in the interfacial method and sometimes employed in the solution polymerization process can be eliminated.
Accordingly, it is the object of this invention to provide a new process for the production of high molecular weight polyesters having a low melt viscosity and an improved yield per volume of reactants and solvents by the solution or homogeneous polymerization technique. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.